Polypropylene compositions with improved mechanical properties have been prepared by adding small amounts of additives to polypropylene. For example, WO 2003/048434 discloses a method for making polypropylene monofilaments by adding to the polypropylene 0.1 to 20% of an additive. EP0080274B1 discloses methods for making polypropylene melt-spun fibers by adding 0.1 to 10% of another polymer that is immiscible in a melt of the polypropylene (in particular, polyhexamethylene adipamide). Polypropylene has also been modified by the addition of small amounts of liquid crystal polymers, polyethylene, polyethylene glycol and nylon 66 (Journal of Applied Polymer Science, 1986, 31(8), 2753-68). Blends of polypropylene and ethylene/alkyl acrylate copolymers have been previously disclosed (see, e.g., U.S. Pat. No. 3,433,573).
Thermoplastic materials are commonly used to manufacture various shaped articles which may be utilized in applications such as automotive parts, food containers, signs, packaging materials and the like. Shaped articles comprising polypropylene (PP) homopolymer may be prepared from the molten polymeric material by a number of melt extrusion processes known in the art, such as injection molding, compression molding, blow molding, profile extrusion and the like.
Shaped articles may also be prepared by thermoforming processes, wherein a thermoplastic film or sheet is heated above its softening temperature and formed into a desired shape. This formed sheet of a film or laminate is usually referred to as a forming web. Various systems and devices are used in a thermoforming process, often accompanied by vacuum-assist and plug-assist components to provide the proper forming of the forming web into a predetermined shape. Thermoforming processes and systems are well known in the art.
Examples of molded articles comprising polypropylene include injection molded or compression molded caps or closures for containers. Containers having closures are well known and have a wide variety of uses. For example, they may contain medicines or pharmaceuticals. They may contain drinks, such as water, milk, carbonated or non-carbonated beverages, and the like, or wines or spirits (e.g. gin or whiskey). They may be used to contain foods. They may also be used to contain petroleum products, oil, or household preparations or chemicals such as hair care products, detergents, bleaches and the like. The present invention is applicable to all of these areas of use and many others besides. The function of a closure or cap is to adequately seal the contents of a container against leakage from or into the container. In many instances, the cap is designed for repeated removal and replacement as the consumer accesses the contents of the container.
Closures or caps for such containers are often prepared from thermoplastic compositions such as PP by injection molding or compression molding. PP caps have been disclosed, for example, in U.S. Pat. Nos. 4,550,843; 4,770,309; 4,807,772 and 5,454,476.
Typically polypropylene caps consist of a top and a depending skirt that close around the neck of the container. In some instances caps may comprise continuous or discontinuous threads that provide screw closures to the container and/or snap closures. They may also incorporate dispensing features, tamper-evidence features and child resistant features. Other decorative or functional features may also be present. They may also include combinations with other materials (e.g., caps having metal lid portions or portions utilizing plastic materials other than polypropylene). The PP caps may be linerless (see U.S. Pat. No. 4,770,309) or have a separate liner that is inserted or compression molded into the PP shell of the cap (see U.S. Pat. No. 4,807,772).
Polypropylene caps preferably have sufficient rigidity and strength to provide structural integrity to the closure during assembly and storage of the container while retaining sufficient flexibility to withstand stresses associated with high speed capping operations. Flexibility is also preferred for providing good sealing to the container such as in certain child resistant cap designs.
A problem with polypropylene caps is an unacceptable rate of fracture cracking. Thus, it is desirable to provide PP compositions and shaped articles produced therefrom, such as caps, with improved mechanical properties such as tensile breaking load, tenacity (tensile breaking stress) and elongation to break.